The present disclosure is related to one-dimensional image compression, such as for bi-level images.
As is well-known, a facsimile machine scans a document line by line and converts each line to alternating black and white dots. The resulting document image is referred to as a bilevel image because each pixel is represented by a single bit and its value may be either 0 to represent a black dot or 1 to represent a white dot or pixel. A combination of run-length encoding and modified Huffman coding has been found suitable to compress such bi-level images. The International Telecommunications Union (ITU, formerly known as Consultative Committee on International Telephone and Telegraphxe2x80x94CCITT) has, therefore, provided a number of standards or specifications suitable to compressing such bi-level facsimile images. See, for example, Hunter, et. al., International Digital Facsimile Coding Standards, Proceedings of the IEEE Vol. 68, No. 7, July 1980, pages 854-857. The CCITT Group 3 is one such standard and is applied to facsimile or xe2x80x9cfaxxe2x80x9d machines. The recommendations for Group 3 has two coding schemesxe2x80x94one dimensional and two dimensional. In the one dimensional scheme, the coding of each scan line is performed independently of any other line. Although this standard was recommended for facsimile machines, the approach is also suitable for coding bi-level document images in various other applications, such as, for example, photocopying machines, scanners, etc.
In photocopying or facsimile systems, the performance of the system may be measured in terms of the number of pages copied per minute. It is difficult to specify copying time per page because this time may vary with the content of the page. However, improving the encoding rate to achieve higher pages per minute is desirable in order to improve performance.